Since the 12.x (luminous) Ceph release, the ceph-mgr daemon is required for
normal operations. The ceph-mgr daemon is an optional component in
the 11.x (kraken) Ceph release.

By default, the manager daemon requires no additional configuration, beyond
ensuring it is running. If there is no mgr daemon running, you will
see a health warning to that effect, and some of the other information
in the output of ceph status will be missing or stale until a mgr is started.

Use your normal deployment tools, such as ceph-ansible or ceph-deploy, to
set up ceph-mgr daemons on each of your mon nodes. It is not mandatory
to place mgr daemons on the same nodes as mons, but it is almost always
sensible.